1. Field of the Invention
The present invention relates to a Surface Acoustic Wave duplexer (hereinafter referred to as a SAW duplexer) employed in a high frequency Radio Frequency (RF) filter in a Surface Acoustic Wave resonance filter employed in small, portable communication equipment such as portable telephones. This application is a counterpart application of Japanese application Serial Number 157563/2000, filed May 29, 2000, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
In recent years, the development of small, lightweight portable communication equipment typified by portable telephones etc. has advanced at a dramatic pace. Small, high-performance duplexers equipped with transmission and receiving filters employed by this kind of communication equipment is therefore also being sought. SAW duplexers constructed using SAW filters as small-type duplexers have also been put forward, and have made a substantial contribution regarding reducing the size of portable communication equipment. Rather than being just small, it is desirable for the SAW duplexers to have low pass band insertion loss and substantial attenuation band attenuation. Therefore, with, for example, SAW filters having a ladder-type circuit configuration of SAW duplexers, there has been proposed a configuration where parallel arm SAW resonators and ground potential are connected together using bonding wire. In this configuration a terminal connected to one end of the parallel arm SAW resonator of a SAW filter chip and a ground terminal of the SAW duplexer package are connected using bonding wiring while mounting the SAW filter chip on the SAW duplexer package. In this way, it is possible to provide an inductance component between the SAW filter and the ground potential, and an amount of attenuation within the attenuation band of the frequency characteristics can be ensured while keeping the SAW duplexer small.
However, it becomes more difficult to provide and regulate the inductance component due to the wire bonding as the device itself for the SAW duplexer, etc., becomes smaller. In particular, taking into consideration the influence of ensuring and regulating the inductance component for a SAW filter having a low-band side pass frequency band, technology for ensuring and regulating a desired inductance component for a SAW filter while keeping the SAW duplexer package itself as small as possible is desired where the frequency characteristics of the SAW duplexer are improved.
The object of the present invention is to bring about a small SAW duplexer while improving frequency characteristics of a SAW duplexer.
In order to achieve the aforementioned object, a surface acoustic wave duplexer of the present invention comprises a first surface acoustic wave filter having a first pass frequency band region, a second surface acoustic wave filter having a second pass frequency band lower than the first pass frequency band and including a series arm surface acoustic wave resonator and a parallel arm surface acoustic wave resonator, a multi-layer package substrate including first to third package substrates, electrode pads formed on the first package substrate, a first ground potential pattern formed on the second package substrate, the first ground pattern being connected to the electrode pads and extending to a central region of the second package substrate and a second ground potential pattern formed on the third package substrate. One end of the parallel arm surface acoustic wave resonator is connected to ground potential via the electrode pads and the first and second ground potential patterns.